Schur Class

In complex analysis, the Schur class is the set of holomorphic functions $f(z)$ defined on the open unit disk $\mathbb = \$ and satisfying $, f(z), \leq 1$ that solve the Schur problem: Given complex numbers $c_0,c_1,\dotsc,c_n$, find a function
:$f(z) = \sum_^ c_j z^j + \sum_^f_j z^j$
which is analytic and bounded by on the unit disk. The method of solving this problem as well as similar problems (e.g. solving Toeplitz systems and Nevanlinna-Pick interpolation) is known as the Schur algorithm (also called Coefficient stripping or Layer stripping). One of the algorithm's most important properties is that it generates orthogonal polynomials which can be used as orthonormal basis functions to expand any th-order polynomial. It is closely related to the Levinson algorithm though Schur algorithm is numerically more stable and better suited to parallel processing.

Schur function

Consider the Carathéodory function of a unique probability measure $d\mu$ on the unit circle $\mathbb =\$ given by

:$F(z) = \int \frac d\mu(\theta)$
where $\int d\mu(\theta) = 1$ implies $F(0)=1$. Then the association
:$F(z) = \frac$

sets up a one-to-one correspondence between Carathéodory functions and Schur functions $f(z)$ given by the inverse formula:
:$f(z) = z^\left( \frac \right)$

Schur algorithm

Schur's algorithm is an iterative construction based on Möbius transformations that maps one Schur function to another. The algorithm defines an infinite sequence of Schur functions $f\equiv f_0,f_1,\dotsc,f_n,\dotsc$ and Schur parameters $\gamma_0,\gamma_1,\dotsc,\gamma_n,\dotsc$ (also called Verblunsky coefficient or reflection coefficient) via the recursion:
:$f_=\frac\frac, \quad f_j(0)\equiv \gamma_j \in \mathbb,$
which stops if $f_j(z)\equiv e^ = \gamma_j \in \mathbb$. One can invert the transformation as
:$f(z)\equiv f_0 (z) = \frac$
or, equivalently, as continued fraction expansion of the Schur function
:$f_0(z)=\gamma_0+\frac$
by repeatedly using the fact that
:$f_j(z)=\gamma_j+\frac.$

See also

* Orthogonal polynomials on the unit circle
* Szegő polynomial

References

{{Reflist

Complex analysis